Method of coating paper



Dec. 24, 1963 H. T. RANDALL Y 3,115,438

METHOD OF' COATING PAPER Filed Aug. 11, 1960 fx 27 @D Q l Q V/.o

HEME/E7' IKA/WALL BY www@ 7%4/ United States Patent O 3,115,438 METHQD Git? SATENG PAPER Herbert T. Randall, Hamilton, (litio, assigner to Champion Papers line., Hamiiton, Ohio, a corporation of hio Filed Aug. 1l, 196i), Ser. No. 49,033 Ciaims. (El. 1621t4l) This invention relates to the manufacture of cast surface coated paper.

In the prior art methods of producing cast coated paper it has been the practice to apply a wet coating material to a previously and separately formed paper sheet followed by drying the coating in intimate contact with a solid casting surface such as a chromium plated drum. Such a process is disclosed in the US. patent to Bradner, No. 1,719,166. Although this process has been successfully practiced commercially for a number of years, it nevertheless suffers from several disadvantages. Gne difliculty is the fact that in order to obtain a high gloss smooth sheet it is necessary to apply an excessively heavy weight of coating either in the casting step itself or in combination with conventional coating operations prior to the casting step. This is because of t. e fact that in the manufacture of the raw stock, defects are often produced such as coclde, fuzz, poor formation and grainy edges, which are difficult to cover. Calendering of such a raw stock to reduce these deiiciencies may result in crushed streaks or blotchy areas. Furthermore, the application of a heavy layer of coating to cover these defects requires a lessening of the casting speed and often results in a weak and brittle sheet having poor fold properties.

A second disadvantage of the casting procedure as heretofore employed is the fact that to a large extent it requires an undue number of steps and an extensive amount of equipment. Thus it has heretofore been the practice to prepare the paper web such as on a Fourdrinier or cylinder machine followed by drying it and winding it up into suitable rolls. When the paper is then cast coated it is desirable to rewet the paper such as on the casting drum in order to avoid a problem of curling. The latter is disclosed in US. patent to Warner, No. 2,360,919. The initial drying on the paper machine is therefore an eXtra operation which, for economical reasons, should desirably be avoided.

A further difliculty which has been encountered in the prior art process of cast coating of paper is the fact that it is difficult to closely control the uniformity of the coating which is applied to the stock. This is for the reason that the stock, being substantially dry, is not uniformly absorbent to the coating.

An object of this invention therefore is an improved method of cast coating by utilizing undried cellulosic web stocks.

In accordance with this invention it has been found that the above noted disadvant "es are overcome by cast coating a wet paper web contai -ing a critical amount of moisture as it comes from the paper forming machine. According to this method a freshly formed paper web is led from the paper machine through an initial moisture reducing section and then placed in contact with an aque ous coating composition containing adhesive and a nely divided pigment material, followed by drying the web against a smooth casting surface.

The FGURE illustrates apparatus suitable for carrying out the process of this invention.

A web of paper l@ is formed in any conventional paper forming machine il. For simplicity the latter is exempliiied as a cylinder machine in the drawing. lt is to be understood that a Fourdrinicr may be used equally as Pice well. A particularly suitable apparatus, owing to its relatively small size and wide versatility, is the Rotoformer unit manufactured by the Sandy Hill Iron and Brass Works of Hudson Falls, New York, and described in Paper Mill News, vol. 82, No. 23, pages 14 and 18. The Rotoformer is particularly advantageous for use in this invention since its Vacuum sections facilitate control of the moisture content of the web. A roll l2 aids in rcmoving the paper web from the paper forming machine. A suction press is illustrated by rolls i3 and le. Th' may suitably be equipped with a vacuum pump to further reduce the water content of the web. A smooth press, as represented by rolls l5 and le, may be provided in order to smooth the web by pressure. The roll 26 is a guide roll. The drums 27 are heated drying drums which also may be employed to reduce the moisture content of the web to the desired level. lt is to be understood that any suitable means may be provided in order to control the moisture content. rlChus, either the suction press or the drying drums in the drawing may be dispensed with, use interchangeably or replaced oy other suitable means provided that the moisture content of the web is properly controlled. Rolls such as 28 may be provided in order to guide the web to the coater lil. Preferably a moisture measuring device is located ahead of the coater to automatically record the moisture content of the web. The coating applicator l@ may be any conventional coater suc'n as a roll coater, an air knife coater, a doctor roll, etc. Roll Ztl merely serves to press the coated web in contact with the heated casting drum 2d. The latter should have a smooth surf-ace and for this reason a polished chromium surface is preferred. Nickel, brass `and hard rubber are surfaces which also may be employed. toll 22 permits removal of the dried coated web from the casting drum and from there it is led to the windup reel 2d equipped with guide rolls 23 and 25. In view of the high water content of the web stock a felt (not shown in the drawing) will ordinarily be employed to support the web throughout all or a portion of its length as it passes from the paper forming machine il until it reaches the casting drum il. Various other supporting carriers, such as a rubber belt, may similarly be used for this purpose.

The coatings employed for producing the cast surface will ordinarily be aqueous dispersions of a finely divided pigment together with an adhesive such as casein, soy protein, starch or a synthetic polymer emulsion. lixamples of pigments include calcium carbonate, clay, titanium dioxide, etc. The solids content of the coating will usually be in the range ot' 2li-65% by weight al though it may contain more or less depending upon the desired thickness of the coating on the web. Usually it is desired to worlt within the range of 45-65% by weight solids content in order to give maximum coverage of 'the sheet.

instead of coating the pper web cas against the drum it is also possible to coat he drum d' rectly with the coating composition and to press we' but uncoated web in Contact with the coated drt-. this way it is possible to decrease substani ally the amount of coating which is necessary to produce a high gloss cast iinish. lt is important that the coating on the drum is not dried to any appreciable extent, i.e., less than 20% water, before being contacted with the paper web or some of the advantages of the more uniform absorption of coating into the web will not be realized.

Still another method of regulating the amount of coating which may be applied to the web involves precoating the freshl formed web prior to application of the coating for tl e casting operation. Thus, referring to the drawing, it is for certain purposes desirable to position a precoatcr between guide rolls and coating applicator il@ or between the paper` machine il and the driers 2.7. Such a precoater can be the same or different from the coating applicator. ne use of this additional coater makes possible the obtaining of sheets with a much higher total weight of coating than is possible by the single coating operation. The precoater shouid preferably employ the same coating which is to be later applied by the cast coater although different coatings of pigment and adhesive may also be used.

The moisture content of the web prior to cast coating must be continuously controlled in order to produce a satisfactory cast coated sheet. It is necessary that the moisture content of the web, between the time of formation and casting on the drum, be maintained at a sufficiently high level. A moisture content of at least about by weight is generally desired for the freshly formed web. lt will not usually be desirable to employ a web containing more than about 65% by weight of water. When the moisture content is greater than 65% adequate drying cannot be achieved a+ reasonable speeds on the casting drum. .M'oreoA/er, with so much water the web is easily torn and makes handling di'lioult. Also, and most important, the quality of the coated sheet drops off sharply when the Water content is more than 65% because of the voids created by the loss of Water. When Vthe moisture content of the sheet is below 20%, the fibers shrink, distort and stitien and it becomes increasingly diihoult to tho-roughly wet the surface of the web with coating. Due to the lack or" absorbency of this relatively low moisture content web, a uniform coating is not readily obtained.

The casting of an undried sheet in accordance with this invention avoids the undesirable characteristics of cockie and grainy edges normally encountered in the preparation of raw stock for a conventional casting operation. in addition the wct casting permits attening of the sheet against the drum surface 'with a minimum amount of pressure and with a minimum coating Weight. l it has been found that the time required to completely dry a web which contains 20-65% by Weight of moisture prior to being coated with an aqueous pigmented coating is not substantially greater than that which is required to dry a relatively moisture free web which has been coated with the same aqueous coating. The reason for this apparently lies in the fact that the web is so uniformly wet throughout its thickness that the Water molecules are easily vaporized. Thus with a 12 foot diameter chromium surface casting drum heated to a temperature of 15G-212 or more it is possible to operate at speeds up to 300 feet per minute. When the moisture content of the web is regulated to Within the particularly desirable range of 45-60% by weight it is possible to operate at speeds even in excess of 300 feet per minute.

`In order to facilitate removal of tie cast sheet from "the casting drum it is desirable to produce a monomoleciular film of an oleaginous release agent on the surface of the drum as disclosed in the US. Reissue Patent to ilt/iontgomery No. 23,637, reissued March 24, i953. The

iilrn of release agent is desirably formed by builing the drum with the oleaginous material and by replenishing The coatings emyable for use in the present invention.

Depending upon the speed of the casting drum and its temperature, the resulting cast sheets, after release from the drum, may s 'il contain appreciable amounts of moisture. Thus a cast sheet can be made to release from the drum when the coating is dry although the web itself may still contain a relatively high amount of moisture, i.e., the total moisture content of the coated web can be in excess of 16% by weight with the bulk of this being present in the web, at east immediately after casting. higher ,amount of moisture in the web makes it possible to control the curling tendency of the paper by subjecting it to a further drying treatment. In addition the higher moisture content of the paper is desirable in certain printing operations.

It has surprisingly been found that cast coating of a freshly formed web in accordance with this invention roduces a quite different and Superior eiect from that obtained by cast coating a relatively dry web or even a Web which has been dried and subsequently rewet. The reason for this apparently resides in the fact that a freshly formed web has a uniform moisture content throughout its thickness which renders it uniformly absorbent toward the coating. When a sheet is once dried of moisture and later revvet, the moisture content throughout the thickness of the sheet is not uniform unless a lengthly period of time is provided in which the moisture content can come to equilibrium. Even then the shrinkage and distortion of the dried fibers cannot be fully corrected. The greater absorbency of a freshly formed web makes possible a sizeable increase in the speed of the coating operation.

rPhe present invention has the additional advantage of permittin f the application of much heavier coatings than has heretofore been possible by the conventional casting techniques. There is no ditliculty in getting 2t) pounds of coating per ream at the same solids content onto a sheet that would have given only l() pounds per roam if the sheet lvere dried. The method of this invention permits the application to a single side of the paper of up to 4() pounds of coating per ream and even more.

By making the casting step a part of the paper making operation it is possible to provide for continuous inspection and for immediate correction of defects. In addition, the losses normally encountered in unwinding and rewinding the rolls and in the handling of the rolls in the extra steps of the conventional process are avoided.

The following exaropies Will serve to illustrate the practice of theinstant invention.

Example l Percent by Weight moisture in web Number of dricrs Sample The Webs were each supplied to a roll coater which applied 21.4r pounds l(dry) to sample (a) and 24.2 pounds (dry) to sample (b) of a coating comprising a mixture of:

Ingredient: Parts by weight Clay (dry) Casein l1 Latex of a polymer of 40% by Weight butadiene and 60% by weight styrene having a total solids content of 50% by Weight 7 Oleic acid l Sufficient Water to dilute the coating to 62% total solids.

The web in each case Was then pressed against a rotating chromium surfaced drum heated to F. which had initially been buffed to establish a film of release agent on its surface. The coated Webs were each easily stripped from the drum when dry to produce an eX- tremely glossy cast surface.

Example Il The procedure of Example I is repeated using a coating of the same formulation. A heavy stock weighing an average of about 110 pounds per ream dry (500 sheets 25 X 38 inches) and containing approximately 45% by weight moisture was taken from the first drier following the Fourdrinier. The coater applied about 14-15 pounds (dry) to sample (a) and about 25 pounds (dry) to sample (b). With sample (a) the chromium drum was heated to 204 F. whereas with sample (b) it was heated to 200 F. Each coated web stripped readily from the drum to produce a high gloss cast sheet when the coating was dry. At this stage sample (a) contained about 1.7% by Weight of Water while sample (b) contained nearly 20% water. Sample (b) was led through a heated drier to further reduce the moisture content to 3%.

Although the preferred embodiments of this invention have been disclosed, it will be obvious that many variations and changes can be made without departing from its intended scope.

I claim:

1. The method of producing cast coated paper which comprises forming a web from a furnish of water and pulp, partially drying said web to reduce tne moisture content to no less than 20% by weight and no more than 65% by Weight, thereafter contacting the web with Cil an aqueous coating composition comprising adhesive and finely divided pigment material, drying said web with said aqueous coating composition against a smooth casting surface and removing the dried coated web from said casting surface.

2. The method of claim l wherein the paper web is coated with the aqueous coating composition and then is pressed into contact with the casting surface.

3. The method of claim l wherein the paper web is pressed against a casting surface which has been coate with the aqueous coating composition.

4. The method of claim 1 wherein the casting surface is a polished chromium surface.

5. The method of claim 1 wherein the coated web is removed from the casting surface before the moisture content of the coated web is reduced below 10%.

References Cited in the iile of this patent UNITED STATES PATENTS Re. 23,637 Montgomery Mar. 24, 1953 2,032,211 Hornbostel Feb. 25, 1936 2,772,604 Muggleton Dec. 4, 1956 2,919,205 Hart Dec. 29, 1959 3,024,160 Kapral Mar. 6, 1962 3,044,896 Warner July 17, 1962 3,054,716 Bergstein et al. Sept. 18, 1962 

1. THE METHOD OF PRODUICING CAST COATED PAPER WHICH COMPRISES FROMING A WEB FROM A FURNISH OF WATER AND PULP, PARTIALLY SAID WEB TO REDUCE THE MOISTURE CONTENT TO NO LESS THAN 20% BY WEIGHT AND NO MORE THAN 65% BY WEIGHT, THEREAFTER CONTACTING THE WEB WITH AN AQUEOUS COATING COMPOSITION COMPRISING ADHESIVE AND FINELY DIVIDED PIGMENT MATERIAL, DRYING SAID WEB WITH SAID AQUEOUS COATING COMPOSITION AGAINST A SMOOTH CASTING SURFACE AND REMOVING THE DRIED COATED WEB FROM SAID CASTING SURFACE. 